Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

"Assimilating the disease to small-pox and to vaccination, it occurred to me that in order to inoculate yellow fever it would be necessary to pick out the inoculable material from within the blood vessels of a yellow fever patient and to carry it likewise into the interior of a blood vessel of a person who was to be inoculated. All of which conditions the mosquito satisfies most admirably through its bite."

In the course of his study of the problem, Finlay made detailed studies of the life history and habits of the common mosquitoes at Havana, and arrived at the conclusion that the carrier of the yellow fever was the Culex mosquito or Aëdes calopus, as it is now known. With this species he undertook direct experimental tests, and believed that he succeeded in transmitting the disease by the bite of infected mosquitoes in three cases. Unfortunately, possibility of other exposure was not absolutely excluded, and the experiments attracted little attention.

Throughout the next twenty years Finlay continued his work on yellow fever, modifying his original theory somewhat as time went on. Among his later suggestions was that in the light of Smith's work on Texas fever, his theory must be "somewhat modified so as to include the important circumstance that the faculty of transmitting the yellow fever germ need not be limited to the parent insect, directly contaminated by stinging a yellow fever patient (or perhaps by contact with or feeding from his discharges), but may be likewise inherited by the next generation of mosquitoes issued from the contaminated parent." He believed that the bite of a single mosquito produced a light attack of the disease and was thus effective in immunizing the patient. Throughout the period, many apparently successful attempts to transmit the disease by mosquitoes were made. In the light of present day knowledge we must regard these as defective not only because possibility of other infection was not absolutely excluded but because no account was taken of the incubation period within the body of the mosquito.

In 1900, while the American army was stationed in Cuba there occurred an epidemic of yellow fever and an army medical board was appointed for "the purpose of pursuing scientific investigations with reference to the acute infectious diseases prevalent on the island." This was headed by Walter Reed and associated with him were James Carroll, Jesse W. Lazear and Aristides Agramonte, the latter a Cuban immune. For a detailed summary of this work the lay reader cannot do better than read Dr. Kelly's fascinating biography "Walter Reed and Yellow Fever."

Arriving at the army barracks near Havana the Commission first took up the study of Bacillus icteroides, the organism which Sanarelli, an Italian physician, had declared the causative agent in yellow fever. They were unable to isolate this bacillus either from the blood during life or from the blood and organs of cadavers and therefore turned their attention to Finlay's theory of the propagation of yellow fever by means of the mosquito. In this work they had the unselfish and enthusiastic support of Dr. Finlay himself, who not only consulted with them and placed his publications at their disposal, but furnished eggs from which their experimental mosquitoes were obtained. Inoculations of eleven non-immunes through the bite of infected mosquitoes were made, and of these, two gave positive results. The first of the two was Dr. Carroll who allowed himself to be bitten by a mosquito which had been caused to feed upon four cases of yellow fever, two of them severe and two mild. The first patient had been bitten twelve days before.

Three days after being bitten, Dr. Carroll came down with a typical case of yellow fever. So severe was the attack that for three days his life hung in the balance. During his convalescence an incident occurred which showed how the theory of mosquito transmission of the disease was generally regarded. We quote from Dr. Kelly: "One of his nurses who came from Tennessee had had considerable experience with yellow fever, having indeed, lost her husband and several children from it. One day early in his illness Dr. Carroll mentioned to her that he had contracted the disease through the bite of a mosquito, and noticed that she looked surprised. Some time later, when well enough to look over the daily records of his condition, he found this entry: 'Says he got his illness through the bite of a mosquito,—delirious'."

The second case was that of an American who was bitten by four mosquitoes, two of which had bitten severe (fatal) cases of yellow fever twelve days previously, one of which had bitten a severe case (second day) sixteen days before and one which had bitten a severe case eight days before. Five days later, the subject developed a well pronounced but mild case of the disease.

In the meantime, another member of the Commission, Dr. Lazear, was accidentally bitten by a mosquito while collecting blood from yellow fever patients. Five days later he contracted a typical case which resulted fatally.

So clear was the evidence from these preliminary experiments that the commission felt warranted in announcing, October 27, 1900, that, "The mosquito serves as the intermediate host for the parasite of yellow fever, and it is highly probable that the disease is only propagated through the bite of this insect."

In order to extend the experimental evidence under conditions which could leave no possibility of infection from other sources, a special experimental sanitary station, named in honor of the deceased member of the Commission, was established in an open field near the town of Quemados, Cuba. Here there were constructed two small buildings known respectively as the "infected clothing building" and the "infected mosquito building."